Ultra low power wake-up circuit

ABSTRACT

An apparatus for selectively enabling power including a power supply, and a device having a controller and an input activated by a user. The controller is selectively powered by the power supply. While the device is in a sleep state, a sensing circuit senses activation of the input by the user and enables the power supply to provide power to the controller in response to the sensed activation of the input by the user. A latch circuit causes the power supply to continue to provide power to the controller. The controller is responsive to the sensed activation of the input by the user for enabling the latch circuit and for disabling the latch circuit, thereby reentering the device into a sleep state.

FIELD OF THE INVENTION

The present invention generally relates to electronic devices having asleep mode. In particular, the invention relates to a method andapparatus for placing an electronic device in a power conserving sleepmode and waking it upon an input from a user.

BACKGROUND OF THE INVENTION

For many years, battery-operated devices have been popular. In earlydevices, the user was required to power down the device or manuallydisconnect the battery from the device via a switch so that the devicedid not drain the battery when not in use.

There are now a number of electronic devices that have been developed tooperate with low power so that completely isolating the batteries via auser-operated switch is not necessary. More recent electronic deviceshave also been designed with a sleep state wherein the microprocessor ofthe device will use a switch to cut power to nonessential elements ofthe device thereby saving additional energy by eliminating the leakagecurrents in those elements. When a user presses a button or otherwiseattempts to use the device, the device wakes up and the processor causespower to be restored to the deactivated elements. Even in their sleepstate, these low power devices still have leakage currents.

Even small leakage currents can significantly impact battery life. Thefollowing Table 1 illustrates one model of shelf life of a 9 voltbattery and a comparable set of AA batteries. TABLE 1 Self TotalCapacity Discharge (mAH) (2% per yr) 9 V  580 mAH 11.6 mAH/yr AA 2700mAH   54 mAH/yr

Table 1 assumes a self discharge rate of 2% per year of the capacity ofa battery. Assuming now that a battery will last a year in a device andthat the device is idle for 16 hours every day, then this self dischargerate is equivalent to an idle current drain of 9.2 microamps in the caseof the AA batteries and 1.99 microamps in the case of the 9V battery.

Modem electronic devices are generally designed to use very little powerwhile idle. This is generally accomplished by use of a sleep state inwhich most components are designed not to conduct any current. However,a CMOS device inherently allows some current flow called leakagecurrent. Leakage currents present a significant impediment to batterylife and energy conservation. The following Table 2 illustrates theamount of power dissipated by a device with different leakage currentsassuming that the battery lasts at least a year in a device and that thedevice is in its sleep state for 16 hours every day. TABLE 2 40 uAleakage 5 uA leakage 1 uA leakage Power % of Power % of Power % of Losstotal Loss total Loss total (mAH/yr) capacity (mAH/yr) capacity (mAH/yr)capacity 9 V 233.6 40.28% 29.2 5.03% 5.84 1.01% AA 233.6  8.65% 29.21.08% 5.84 0.22%

As shown in Table 2, leakage currents can present a significant energydrain during a sleep state and impact battery life. For theabove-described usage scenario, almost half of the capacity (40.28%) ofa 9V battery is dissipated by a device with only 40 microamps of leakagecurrent in its sleep state. Reducing the leakage current to 1 microampmeans that at the end of the year, the battery has retained over 39% ofits capacity that it would have otherwise lost to leakage currents.Additionally, since only 1% of the battery's capacity is lost to leakagecurrents during the device's sleep state, the battery's own selfdischarge rate (approximately 2% per year) becomes a relativelyimportant factor in the life of the battery. Thus, it is highlydesirable to decrease the sleep state current draw of these devices toimprove battery life and/or generally conserve energy. Morespecifically, it is desirable to substantially eliminate leakagecurrents within the device while it is in its sleep state, and still beable to easily and quickly wake the device from its sleep state.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, an apparatus is providedfor selectively enabling power. The apparatus includes a power supplyand a device having a controller and an input activated by a user. Thecontroller is selectively powered by the power supply. A sensing circuitsenses activation of the input activated by the user and enables thepower supply to provide power to the device's controller in response tothe sensed activation of the input by the user. The controller isresponsive to the sensed activation of the input by the user forenabling a latch circuit and for subsequently disabling the latchcircuit. The latch circuit causes the power supply to continue toprovide power to the controller while it is enabled.

In accordance with another aspect of the invention, a method is providedfor selectively enabling a power supply of a device having a controllerselectively powered by the power supply. The activation of an input by auser of the device is sensed. The power supply is enabled to providepower to the controller in response to the sensed activation of theinput by the user. The enabled power supply is latched to provide powerto the controller during operation of the device. The power supply isdisabled to discontinue providing power to the controller.

In accordance with another aspect of the invention, an apparatus isprovided for selectively enabling power. The apparatus includes a powersupply and a device having a controller and an input activated by auser. The controller is selectively powered by the power supply. Theapparatus also includes a sensing circuit for sensing activation of theinput by the user and for enabling the power supply to provide power tothe controller in response to the sensed activation of the input by theuser. The controller is responsive to the sensed activation of the inputby the user for causing the sensing circuit to continue enabling thepower supply to provide power to the controller during operation of thedevice and for causing the sensing circuit to disable the power supplyfrom providing power to the controller.

Alternatively, the invention may comprise various other methods andapparatuses.

Other objects and features will be in part apparent and in part pointedout hereinafter.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a wakeup circuit using a passiveswitch sensing circuit and a latch circuit according to one embodimentof the invention.

FIG. 2 is a block diagram illustrating a wakeup circuit using an activeswitch sensing circuit that eliminates the separate latch circuitaccording to one embodiment of the invention.

FIG. 3 is a schematic diagram illustrating a momentary switch andpassive switch sensing circuit according to one embodiment of theinvention.

FIG. 4 is a schematic diagram illustrating a single pole switch andpassive switch sensing circuit according to one embodiment of theinvention.

FIG. 5 is a schematic diagram illustrating a latch circuit according toone embodiment of the invention.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention is an ultra low power wake-upcircuit which draws essentially no power while waiting for a user input.Power is supplied to components of the wake-up circuit directly from apower supply and power is selectively supplied to device componentsthrough a load switch. The wake-up circuit is adapted to wake up thedevice in response to any number of events, such as a user pushing anyof the input keys on the device's user interface, removing or replacinga component from its docking position on the device, receiving a signalfrom another device, or opening or closing the cover of the device. Forexample, in the case of a clinical thermometer such as a predictive orinfrared thermometer, the device should have the capability to wake upwhenever the temperature probe is removed or replaced, whenever theprobe cover is removed or replaced, whenever a button on the device ispressed, or whenever a device cover is opened or closed. The thermometerhas a microprocessor responsive to the inputs from the buttons and theprobe and performs functions in response to these inputs. In someembodiments, after the thermometer has performed its function, themicroprocessor waits for a preset time and instructs the wake-up circuitto place the device in its sleep state. In general, the function of thedevice, the events that wake it up, and the events that return it to itssleep state are based on system-defined requirements and such activitiescan be hardwired or manual. One skilled in the art will recognize thatthe invention is also applicable to pulse meters, blood pressuremonitors, predictive thermometers, blood sugar monitors, and otherelectrical devices.

The following discussion is focused on reducing power consumption ofbattery-powered devices so that their batteries have a longer life. Oneskilled in the art will notice that the invention is equally applicableto devices with other types of power supplies such as AC to DCconverters, AC sources, and other power sources to reduce powerconsumption.

Referring now to FIG. 1, an embodiment of the invention using a passiveswitch-sensing circuit and separate latch circuit is illustrated. Apower supply 102 continuously supplies power to a latch circuit 104, aswitch 106, a passive switch sensing circuit 110, and a load switch 108,all of which, optionally, may be configured to have substantially noleakage currents while waiting for a user input.

In one embodiment, this can be accomplished by designing the circuitsand switches such that they are substantially free of CMOS componentsand rely instead on only NMOS devices. The power supply 102 may bebatteries or any other power device such as an AC/DC power converter.Additionally, the switch 106 may be an array of user input switches (seeFIG. 3).

The passive switch-sensing circuit 110 detects when a user actuatesswitch 106 of the device and provides a SENSE ENABLE signal of a presetduration to enable the load switch 108 to connect the power supply 102and a microprocessor 112 via the load switch 108 and to connect anyother device components such as memory and/or a communications component(not shown) to the power supply 102. In response to receiving power, themicroprocessor 112 activates the latch circuit 104 by a LATCH signal andperforms functions based on user input IN from the actuated switch 106.The activated latch circuit 104 provides a LATCH ENABLE signal to theload switch 108 to continue supplying power to the device's componentsincluding the microprocessor 112. When the device has finished itsoperations, the microprocessor 112 waits for more user input for apreset time period (e.g. a timeout period set by the user) and thensignals the latch circuit 104 via the LATCH signal to place the deviceback into its sleep state. For example, the microprocessor 112 maydiscontinue providing the LATCH signal to the latch circuit 104. Thelatch circuit 104 signals the load switch 108 (e.g., discontinuesproviding the LATCH ENABLE signal) to stop supplying power to thedevice. The load switch 108 then changes state to discontinue supplyingpower VSW to the microprocessor 112. The device thus returns to itssleep state wherein power VPS is only supplied to the user input switch106, passive switch sensing circuit 110, latch circuit 104, and the loadswitch 108. It remains in this sleep state waiting for user input viaswitch 106.

In operation, a user presses a switch 106 on the device to begin use ofthe device. The passive switch sensing circuit 110 senses the change ofstate of switch 106 and temporarily produces the SENSE ENABLE signalcausing the load switch to provide power VSW to the microprocessor 112.In response to the power, the microprocessor 112 starts up and providesthe LATCH signal to the latch circuit. In response to the LATCH signal,the latch circuit provides the LATCH ENABLE signal to the load switchwhich continues to maintain load switch in its enabled state, causing itto continue supplying power from the power supply 102 to themicroprocessor 112. The user can then use the device for its intendedpurpose. When the device has finished its operations, the microprocessor112 waits for further user input for a preset time. If no such input isreceived within a time-out period, the microprocessor 112 terminates theLATCH signal which causes the latch circuit to terminate the LATCHENABLE signal, and the load switch stops supplying power VSW to themicroprocessor 112.

Referring now to FIG. 2, an embodiment of the invention using an activeswitch sensing circuit is shown. In this embodiment, a power supply 202continuously supplies power VPS to a user input switch 206, an activeswitch sensing circuit 210, and a load switch 208, all of which,optionally, may be configured to have substantially no leakage currentswhile waiting for a user input.

In one embodiment, the active switch sensing circuit 210 issubstantially a microprocessor from Microchip Corporation, PIC 10F202,which operates on less than 1 microamp of current while waiting for aswitch 206 input. Some common ancillary circuitry may be required withthis particular component to buffer the switch 206 input. Themicroprocessor used in the active switch sensing circuit 210 has limitedcapabilities which allow it to be designed to have low leakage currents.The active switch sensing circuit 210 is different from the passiveswitch sensing circuit 110 of FIG. 1, in that most of the sensingcircuit's functions are located in one device instead of a number ofdiscrete components. This allows for a smaller overall product andpossibly reduces manufacturing costs.

When a user actuates the input switch 206, the active switch-sensingcircuit 210 temporarily enables the load switch 208 to supply power VSWto the device's microprocessor 212. The microprocessor 212 starts up andinstructs the active switch-sensing circuit 210 to continue causing theload switch 208 to supply power (e.g., provides a LATCH signal). Thedevice then performs operations based on the user input of the switch206. When the device has finished performing its operations, themicroprocessor 212 waits for further user input for a preset period andthen instructs the active switch-sensing circuit 210 to place the deviceinto its sleep state. In response, the active switch-sensing circuit 210signals the load switch 208 via the ENABLE signal to discontinue powerVSW to the microprocessor 212. The device is thus returned to its sleepstate wherein power VPS is only being supplied to the user input switch206, active switch-sensing circuit 210, and the load switch 208. In thisembodiment, the need for the separate latch circuit 104 has beeneliminated by using an active switch-sensing circuit 210 whichincorporates the function of the latch circuit 104.

It is contemplated that the timeout function (e.g. a period of timeduring which the microprocessor waits for further user input for apreset period and then instructs the wake-up circuit to place the deviceinto its sleep state) may be implemented in the microprocessor of thedevice, in a separate circuit of the device, by the wake-up circuititself or by a combination thereof.

A device can, and usually will, have multiple user input switches. Itshould be apparent to one skilled in the art that the invention willwork with all, a select few, or even just one user input switch of thedevice. For example, additional switches are shown in phantom in FIG. 3.

The load switch also has multiple embodiments. In one preferredembodiment, it is a single p-channel FET. It may also be an array ofp-channel FETs which may be required for devices that draw higheramounts of current when in use. In another embodiment, it may be anactive load switch. One such active load switch is an FDC6323manufactured by Fairchild Semiconductor. Active load switches may alsobe arrayed for additional current capacity.

It should be apparent to one skilled in the art that numerouscombinations of sensing circuits, load switches, and user input switchescan be made without deviating from the invention.

Referring now to FIG. 3, one embodiment of a user input switch 106 isshown, along with a passive switch sensing circuit 110. The input VPS isa supply of continuous power from the power supply 102 (e.g., nominally4.5 volts in the case of 3 AAA batteries in series). When a useractivates a momentary switch 302, the SENSE ENABLE signal is pulledelectrically low. When the switch 302 is released, the SENSE ENABLEsignal remains temporarily low for a period of time determined by thevalues of resistor R1 and capacitor C1. The SENSE ENABLE signalactivates the load switch 108 and thus causes power to be supplied tothe device's microprocessor 112 while the signal is present. Theresistor R1 and capacitor C1 of the passive switch sensing circuit 106are selected so that the microprocessor 112 has adequate time to startupand provide the LATCH signal. Example values of resistor R1 andcapacitor C1 for providing a sufficiently long SENSE ENABLE signal whilekeeping power usage low, are 1 megaohm and 0.1 microfarad, respectively.The microprocessor 112 then reads the input signal IN and performsoperations according to that and any other inputs. As shown in phantom,additional sensing circuits may be used to monitor additional inputs.

Referring now to FIG. 4, an alternative embodiment of a user inputswitch 106 is shown, along with a passive switch-sensing circuit 110(FIG. 1). Whereas the switch in FIG. 3 is a momentary switch, a switch402 in this embodiment is a single throw, single pole switch. The inputVSW is a supply of switched power directly from the load switch 108 or aregulator powered or enabled by the switched power from the load switch108, and the input VPS is a supply of continuous power from the powersupply 102 or an associated regulator. The switch 402 is suitable forconnection as an indicator of a component or cover status of a device;e.g., whether the component is in a holder or not, whether there is acover on a probe or not, and whether a cover is open or closed. Anoutput pulse PR instructs the device's microprocessor 112 that thecircuit was awakened by actuation of the associated component. An outputsignal PROBE tells the microprocessor 112 whether the switch 402 is openor closed, thus indicating the status of the component or cover.Changing the state of the switch 402 from open to closed wakes thedevice from its sleep state. The resistor R2 and capacitor C2 should bechosen so that the SENSE ENABLE signal is provided for a sufficientamount of time to start up the device's microprocessor 112. Examplevalues of resistor R2 and capacitor C2 for providing a sufficiently longSENSE ENABLE signal while keeping power usage low, are 1 megaohm and 0.1microfarad, respectively. The microprocessor 112 then supplies the LATCHsignal to the latch circuit 104 which, in turn, causes the load switch108 to continue supplying power to the microprocessor 112. The device isthus awakened from its sleep state and the microprocessor 112 cananalyze to determine status, for example, if a cover has been placed ona probe through the use of the switch 402 and passive switch-sensingcircuit 110.

Referring now to FIG. 5, one embodiment of a latch circuit 104 is shown.While the latch circuit 104 is in its sleep state, both FET 502 and 504are open circuits. The latch circuit 104 thus conducts substantially nocurrent while the device is awaiting a user input, and the LATCH ENABLEsignal is electrically high (e.g., the same voltage as VPS).

When the wake-up circuit has sensed a user input, the microprocessor 112provides an input signal LATCH to the latch circuit 104. The LATCHsignal causes the FET 502 to conduct current. This, in turn, raises thevoltage at the gate of FET 504 causing it to conduct current and theLATCH ENABLE signal is pulled electrically low causing the load switch108 to continue supplying power to the microprocessor 112. After thedevice has finished its operations, the microprocessor 112 waits for atimeout period then discontinues the LATCH signal. The latch circuit 104discontinues the LATCH ENABLE signal which causes the load switch 108 todiscontinue power to the microprocessor 112, placing the device in itssleep state.

It is contemplated that the embodiments in FIGS. 3-5 may be built intothe circuitry of a device, or implemented as an add-on to existingdevice circuitry, such as in a daughterboard configuration.

Having described the invention in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

The order of execution or performance of the methods illustrated anddescribed herein is not essential, unless otherwise specified. That is,it is contemplated by the inventors that elements of the methods may beperformed in any order, unless otherwise specified, and that the methodsmay include more or less elements than those disclosed herein. Forexample, it is contemplated that executing or performing a particularelement before, contemporaneously with, or after another element iswithin the scope of the various embodiments of the invention.

When introducing elements of the present invention or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above products and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

1. An apparatus for selectively enabling power comprising: a powersupply a device having a controller and an input activated by a user,said controller selectively powered by the power supply; a sensingcircuit for sensing activation of the input by the user and for enablingthe power supply to provide power to the controller in response to thesensed activation of the input by the user; a latch circuit for causingthe power supply to continue to provide power to the controller; andsaid controller responsive to the sensed activation of the input by theuser for enabling the latch circuit and for disabling the latch circuit.2. The apparatus of claim 1 further comprising: a load switch responsiveto a power supply enable signal for supplying power to the controller;wherein the sensing circuit initially provides the power supply enablesignal to the load switch in response to the sensed activation of theinput by the user; and wherein the latch circuit provides the powersupply enable signal to the load switch while the latch circuit isenabled by the controller and discontinues providing the power supplyenable signal when disabled by the controller.
 3. The apparatus of claim2 wherein the load switch is an array of load switches.
 4. The apparatusof claim 2 wherein the power supply continuously provides power to theinput activated by a user, the sensing circuit, and the latch circuitindependent of the load switch.
 5. The apparatus of claim 1 wherein theinput activated by a user is a switch associated with a probe forproviding input to the controller.
 6. The apparatus of claim 1 whereinthe input activated by a user is a switch associated with a cover forthe device.
 7. The apparatus of claim 1 wherein the device is a clinicalthermometer, pulse meter, blood pressure monitor, predictivethermometer, or blood sugar monitor.
 8. A method of selectively enablinga power supply of a device having a controller selectively powered bythe power supply comprising: sensing the activation of an input by auser of said device; enabling the power supply to provide power to thecontroller in response to the sensed activation of the input by theuser; latching the enabled power supply to provide power to thecontroller during operation of the device; and disabling the powersupply to discontinue providing power to the controller.
 9. The methodof claim 8 wherein sensing the activation of an input by a user of saiddevice comprises sensing the position of a switch of the device.
 10. Themethod of claim 9 wherein enabling the power supply to provide power tothe controller in response to the sensed activation of the input by theuser comprises initially enabling the power supply.
 11. The method ofclaim 10 wherein latching the enabled power supply to provide power tothe controller during operation of the device comprises providing anenabling signal during operation of the controller.
 12. The method ofclaim 11 wherein disabling the power supply to discontinue providingpower to the controller after operation of the device has endedcomprises discontinuing providing the enabling signal.
 13. An apparatusfor selectively enabling power comprising: a power supply; a devicehaving a controller and an input activated by a user, said controllerselectively powered by the power supply; a sensing circuit for sensingactivation of the input by the user and for enabling the power supply toprovide power to the controller in response to the sensed activation ofthe input by the user; and said controller responsive to the sensedactivation of the input by the user for causing the sensing circuit tocontinue enabling the power supply to provide power to the controllerduring operation of the device and for causing the sensing circuit todisable the power supply from providing power to the controller.
 14. Theapparatus of claim 13 wherein the sensing circuit comprises a sensingmicroprocessor.
 15. The apparatus of claim 13 further comprising: a loadswitch responsive to a power supply enable signal for providing power tothe controller; and wherein the sensing circuit provides the powersupply enable signal to the load switch in response to the sensed inputby the user and/or in response to input from the controller.
 16. Theapparatus of claim 15 wherein the load switch is an array of loadswitches.
 17. The apparatus of claim 15 wherein the power supplycontinuously provides power to the sensing circuit and the inputactivated by a user independent of the load switch.
 18. The apparatus ofclaim 13 wherein the input activated by a user is a switch associatedwith a probe for providing input to the controller.
 19. The apparatus ofclaim 13 wherein the input activated by a user is a switch associatedwith a cover for the device.
 20. The apparatus of claim 13 wherein thedevice is a clinical thermometer, pulse meter, blood pressure monitor,predictive thermometer, or blood sugar monitor.